Laser driver circuits, which are typically used to drive laser diodes, vertical cavity surface emitting lasers (VCSELs) and/or physical media dependent (PMD) lasers, include duty cycle adjustment (DCA) functionality implemented in hardware, circuits and/or software. The DCA functionality enables adjustment of the duty cycle, or equivalently the eye crossing pattern of signals output from a laser driver circuit.
One type of known DCA arrangement includes open loop control of the driver duty cycle (i.e., the arrangement does not include a feedback path). However, as readily appreciated by those having ordinary skill in the art, open loop control systems may not control the driver duty cycle as accurately as desired.
Closed loop control is another known DCA arrangement. One type of closed loop DCA includes a feedback tapped directly from an output driver stage that is coupled to the laser. However, such an arrangement is only useful when a laser is alternating current (AC) coupled to the output of the driver (e.g., the driver output is coupled to the laser through a capacitor) and when the driver includes an on-chip termination.
Systems including lasers that are directly coupled (i.e., not AC coupled) may use a closed loop DCA, such as that shown in a known driver system 100 of FIG. 1. The known driver system 100 includes first and second driver stages 102, 104 that are cascaded to an output stage 106. An input signal is provided to the first driver stage 102 and a driving signal is produced by the output stage 106 and is directly coupled to a laser 108 and a termination 110. The driver system 100 includes a DCA circuit 112 that receives a signal from the output of the second driver stage 104 and produces an output that is coupled to the input of the first driver stage 102. The first and second driver stages 102, 104, the output stage 106 and the DCA circuit 112 are disposed on a chip 114 on which the laser 108 and the termination 110 are not located. Accordingly, the laser 108 and the termination 110 are referred to as being located “off-chip.”
The driver system 100 of FIG. 1 derives feedback from the output of the second driver stage 104. This is necessary because the output stage 106 is directly coupled to the laser 108 and the termination 110 and, therefore, the differential signals produced by the output stage 106 are asymmetrical because one of the output signals is coupled to the laser 108, which may be capacitive or inductive in nature, and the other output signal is coupled to the termination 110, which is not capacitive or inductive. Asymmetrical signals cannot be used as feedback signals to the DCA circuit 112. Accordingly, to ensure symmetry of the feedback signal, the output of the second driver stage 104 is used to generator feedback. However, a significant drawback of using the output of the second driver stage 104 as a source of feedback is that any offset in duty cycle introduced by the output stage 106 cannot be compensated for by the DCA circuit 112 because the feedback signal does not include any such offset, thereby adversely affecting the duty cycle control of the system 100.